1. Field of the Invention
The present invention relates to a heat-sensitive recording material improved in storage stability. In particular, the present invention relates to a heat-sensitive recording material having a heat-sensitive recording layer containing a specified resorcylic anilide derivative added thereto to thereby not only ensure excellent coloring sensitivity but also improve the resistance to heat, light and oil during storage.
2. Description of Related Art
Heat-sensitive recording materials are produced by applying a usually colorless or light-colored coupling substance and a developer which colors the coupling substance upon being heated together with a sensitizer, a binder and other additives to the surface of a support such as paper, synthetic paper, plastic film or sheet. When a heating element such as a thermal head or hot pen is brought into contact with the recording material in a recording device, the coupling substance reacts with the developer to develop a color such as black, thereby forming a record. The above recording materials (heat-sensitive recording materials) are widely used not only in copying books and documents but also in instrumental recorders, computers, facsimiles, telex devices, automatic passenger ticket vending machines, prepaid cards, labels, etc., because they are superior to other recording materials in that time-consuming treatments such as development and fixing can be dispensed with, the records can be obtained in a short time with the use of a relatively simple apparatus, the noise and the environmental pollution are only slight, and they are inexpensive.
With respect to the conventional heat-sensitive recording materials, those practically satisfactory from the viewpoint of initial coloring sensitivity and stain of non-image areas (fogging of non-image areas) are obtained by an appropriate combination of a coupling substance (leuco dye), a developer which effects thermal coloring of the coupling substance, and an optionally employed sensitizer.
However, when the above heat-sensitive recording materials are exposed to sunlight or illuminating rays for a prolonged period of time, a drawback occurs such that the printed area (colored area) fades or vanishes while the non-image area discolors (yellows). When a printout from a facsimile, a word processor or a personal computer is allowed to stand still on a desk, the recorded image becomes unclear, thereby causing a problem on document storage.
Further, the conventional heat-sensitive recording materials have drawbacks in that not only is the light resistance poor but also the storage stability is lowered when they are stored under severe conditions, for example, at high temperatures, when fingerprints are impressed thereon or when the recording material is brought into contact with oils such as a plasticizer migrated from, for example, a desk mat made of polyvinyl chloride or the like. Therefore, there has been a strong demand for the improvement of the stability of the heat-sensitive recording material during storage.
Among the above developers, it is known that hydroxybenzoic amide compounds have relatively high storage stability. For example, additions of salicylicamide compounds for improving the coloring sensitivity, the resistance of colored images to plasticizers and the stability of non-image areas on the recording material have been proposed in, for example, Japanese Patent Laid-Open Nos. 11036/1978, 49294/1983 and 140096/1984. Despite the use of such compounds, however, the stabilities of the recording materials during storage have been still unsatisfactory and have been unable to satisfy the practical requirements at all.
Japanese Patent Laid-Open No. 209590/1983 proposes to use a developer comprising a resorcylic acid derivative having two hydroxyl groups, described examples thereof including a resorcylic anilide. Although this anilide achieves a significant improvement in the stability of colored images as compared with the salicylicamide compound, it has a drawback in that the fogging of non-image areas on the recording material is intense and especially intense after the storage of the recording material. Therefore, the resorcylic anilide is unsatisfactory from the viewpoint of practical applications.